1. Field of the Invention
The present invention relates to fluidized beds, the sizing of inlet holes for the introduction of a fluidizing medium into a fluidized bed and methods of fluidizing. In another aspect, the present invention relates to fluidized bed reactors and to methods of gasification. In even another aspect, the present invention relates to fluidized bed coal and carbonaceous material gasification and to methods of coal and carbonaceous material gasification.
2. Description of the Related Art
Fluidization is commonly defined as an operation by which particulate fine solids are transformed into a fluid-like state through contact with a gas or liquid. Fluidized beds are known for their high heat and mass transfer coefficients, due to the high surface area-to-volume ratio of particulate matter to fluidizing medium. Fluidized beds are used in a wide variety of industrial processes such as chemical reactions, catalytic reactions, classifying, drying, mixing, granulation, coating, heating and cooling.
In many industrial applications, a fluidized bed consists of a vertically-oriented column filled with granular material with a fluid (gas or liquid) being pumped upwards through a distributor at the bottom of the bed. When the drag force of flowing fluid exceeds gravity, particles are lifted and fluidization occurs.
In a chemical reaction process, a fluidized bed suspends solids on upward-blowing air or liquid. The result is a turbulent mixing of gas or liquid and solids. The tumbling action, much like a bubbling fluid, provides more effective chemical reactions and heat transfer.
Fluidized bed technology is utilized in coal gasification. There are a number of patent applications that are directed toward fluidized beds and/or coal gasification.
A coal gasification reactor of the type wherein agglomerated coal ash is withdrawn from a fluid reaction bed of finely divided coal without the removal of the finely divided coal particles is disclosed in Jequier et al, U.S. Pat. No. 2.906.608 and Matthews et al. U.S. Pat. No. 3,935,825. These patents are incorporated herewith by reference.
In a coal to gas conversion process of the type referenced, a vessel is provided for a fluidized bed. A gas distribution grid is usually positioned in the vessel and defines the bottom surface of the fluidized bed. The central portion of the grid may be conical or cylindrical in shape and comprises a passage. At the bottom of the passage, a constriction is provided having a fixed opening defining a venturi of fixed throat size to provide a uniform upward gas velocity into the vessel and thus into the fluidized bed. Directing a stream of high velocity gas through the venturi or passage into the reaction vessel causes ash particles in the vessel agglomerate and eventually discharge through the passage and venturi throat.
U.S. Pat. No. 4,023,280, issued May 17, 1977, to Schora et al., discloses a fluidized bed of material retained in a vessel which receives a high velocity gas stream through a venturi orifice and passage to assist in the agglomeration of ash particles. The particles form a semi-fixed bed within the passage upstream from the venturi orifice. The particular dimensions of the semi-fixed bed are dependent, in part, upon the orifice size of the venturi. An iris valve defining the orifice permits adjustment of the cross-sectional area of the orifice and thereby controls the velocity of the gas stream through the venturi.
U.S. Pat. No. 4,435,364, issued Mar. 6, 1984, to Vorres, discloses an apparatus for withdrawing agglomerated solids, e.g. ash, from a fluidized bed of finely divided solid hydro-carbonaceous material, e.g. coal, is described. Agglomeration is effected by a high temperature reaction between the inorganic constituents of the hydro-carbonaceous material in the fluidized bed environment. A venturi is utilized to serve as a passage for withdrawing the agglomerated solids from the fluidized bed. Spiral or other descending ridges are positioned on the interior surface of the constricted cylindrical opening of the venturi to permit variable and increased rates of agglomerate discharge with improved separation and classification of the solid materials.
U.S. Pat. No. 4,453,495, issued, Jun. 12, 1984, to Strohmeyer, Jr., discloses an integrated control for a steam generator circulating fluidized bed firing system. The system includes an integrated control means, particularly at partial loads, for a steam generator having a circulating fluidized bed combustion system wherein gas recirculation means is used to supplement combustion air flow to maintain gas velocity in the circulation loop sufficient to entrain and sustain particle mass flow rate at a level required to limit furnace gas temperature to a predetermined value as 1550 F. and wherein gas recirculation mass flow apportions heat transfer from the gas and recirculated particles among the respective portions of the steam generator fluid heat absorption circuits, gas and circulating particle mass flow rates being controlled selectively in a coordinated manner to complement each other in the apportionment of heat transfer optimally among the fluid heat absorption circuits while maintaining furnace gas temperature at a predetermined set point.
U.S. Pat. No. 4,454,838, issued Jun. 19, 1984, to Strohmeyer, Jr., discloses a dense pack heat exchanger for a steam generator having a circulating fluidized bed combustion system whereby a bed of solid particles comprising fuel and inert material is entrained in the furnace gas stream. Means are provided for collecting high temperature bed solid particles downstream of the furnace. The dense pack heat exchanger directs the hot collected particles down over heat transfer surface, such surface being a portion of the steam generator fluid circuits. Flow is induced by gravity means. The dense compaction of the solid particles around the fluid heat exchange circuits results in high heat transfer rates as the fluid cools the compacted solid material. The heat exchange surface is arranged to facilitate flow of the solid particles through the heat exchanger.
U.S. Pat. No. 4,462,341, issued Jul. 31, 1984, Strohmeyer, Jr. discloses a steam generator having a circulating fluidized bed combustion system whereby there is provision to admit air flow incrementally along the gas path to control combustion rate and firing temperature in a manner to maintain differential temperatures along the gas path. The initial portion of the gas path where combustion is initiated can be held in one temperature range as 1550 F. which is optimum for sulphur retention and the final portion of the combustion zone can be elevated in temperature as to 1800 F. to produce a greater degree of heat transfer through the gas to fluid heat exchange surface downstream of the combustion zone.
U.S. Pat. No. 4,745,884, issued May 24, 1988, to Coulthard, discloses a fluidized bed steam generating system includes an upstanding combustion vessel, a gas/solids separator, a convection pass boiler and a heat exchanger positioned directly below the boiler and all of the above elements except the gas/solids separator are enclosed within a waterwall structure having outside waterwalls and a central waterwall common to the reactor vessel on one hand and the convection pass boiler and heat exchanger on the other hand. The close proximity of the components of the system eliminate numerous problems present in conventional multi-solid fluidized bed steam generators.
U.S. Pat. No. 5,082,634, issued Jan. 21, 1992, to Raufast, discloses a fluidized bed apparatus comprising a fluidization grid arranged in the lower part of this apparatus, this grid being provided at its center with a circular aperture communicating with a discharge chamber and occurring in the form of a surface of revolution consisting of the joined lateral surfaces of at least two coaxial truncated cones of revolution, virtual vertices of which are oriented downwards.
In spite of all of the advancements in fluidized bed technology, one problem that may be encountered, is that of uneven flow of the fluidizing medium through the fluidized bed and through the inlet holes in the injection grid or other device for injecting fluidizing medium, especially if the injection grid is sloped and different fluid heads exist above the various inlet holes.